USP36: A Key Deubiquitinase in Ribosome Biogenesis and Cell Fate Regulation

Molecular Structure and Subcellular Localization of USP36

USP36, a critical member of the deubiquitinase family, exhibits unique structural features and distinct subcellular localization patterns. Genomically, the USP36 gene is located on human chromosome 17q25.3 and encodes a large protein of 1,124 amino acids. Unlike other USP family members, USP36 contains a prominent nuclear localization signal (NLS) and a nucleolar localization signal (NoLS) at its N-terminus, dictating its primary subcellular distribution. X-ray crystallography reveals that USP36's catalytic core adopts the classic USP fold, consisting of "thumb," "palm," and "finger" subdomains that form a deep groove for ubiquitin binding. Notably, USP36's C-terminal region harbors multiple acidic amino acid repeats, which mediate interactions with ribosomal proteins and nucleolar RNAs, providing a structural basis for its role in ribosome biogenesis.

Subcellularly, USP36 exhibits high regional specificity. Immunofluorescence and electron microscopy confirm its predominant enrichment in the nucleolus, particularly co-localizing with fibrillar centers and dense fibrillar components—a pattern aligning with the spatial organization of ribosomal DNA transcription and early ribosome assembly. Live-cell imaging reveals dynamic changes in USP36's nucleolar localization during the cell cycle: uniform distribution across nucleolar subregions in G1 and S phases, dispersion into the nucleoplasm during mitotic prophase as the nucleolus disassembles, and rapid re-localization to reforming nucleoli in daughter cells. These cyclical changes reflect USP36's tight association with nucleolar dynamics.

USP36 expression also displays tissue specificity and stress responsiveness. Northern blot analyses show higher USP36 levels in metabolically active tissues (e.g., liver, pancreas, lymphoid tissues) compared to muscle or neural tissues. Under stress conditions like nutrient deprivation or translational inhibition, USP36 expression increases 2- to 3-fold, likely mediated by the mTOR pathway. Intriguingly, during viral infections (e.g., influenza), USP36 redistributes from the nucleolus to the nucleoplasm, possibly due to viral hijacking of host translation machinery. These dynamic localization and expression changes suggest USP36 as a key adaptor to environmental cues.

USP36's Central Role in Ribosome Biogenesis

USP36 is indispensable for ribosomal RNA (rRNA) processing and subunit assembly. It stabilizes ribosome biogenesis factors (e.g., fibrillarin, RPL5, RPL11) via deubiquitination. Mass spectrometry identifies physical interactions between USP36 and multiple components of the 90S pre-ribosome, dependent on its C-terminal acidic region. Functionally, USP36 knockout cells show ~40% reduced 47S pre-rRNA processing efficiency, impairing mature 18S, 5.8S, and 28S rRNA production—a defect rescued by wild-type but not catalytically inactive USP36.

USP36's role in ribosomal protein quality control ensures precise ribosome maturation. It selectively deubiquitinates correctly assembled ribosomal proteins while allowing misfolded/mislocalized ones to be degraded by the proteasome. This "quality-check" mechanism ensures only structurally intact subunits advance. Proteomic comparisons reveal that USP36-deficient cells exhibit 2- to 5-fold increased ubiquitination of at least 15 ribosomal proteins, shortening their half-lives and causing assembly defects. This explains why USP36 knockout cells show 30–40% reduced translation efficiency.

USP36 uniquely modulates the ribosomal stress response. When ribosome biogenesis is impaired, USP36 is phosphorylated and enhances binding to free ribosomal proteins, preventing their ubiquitination and subsequent p53 activation. This allows cells to prioritize ribosome repair over apoptosis under mild stress (e.g., low-dose cycloheximide). However, under severe/prolonged stress, USP36 itself is hyperphosphorylated by CK2 kinase and inactivated, permitting full p53 pathway activation. This biphasic regulation enables context-appropriate cell fate decisions.

USP36's Complex Role in Cell Cycle Regulation

USP36 multi-facetedly regulates G1/S transition. It stabilizes pro-proliferative factors like cyclin D1 and E2F1 via deubiquitination. Co-immunoprecipitation confirms USP36-cyclin D1 complexes form in late G1, dependent on cyclin D1 Thr286 phosphorylation. In breast cancer cells, USP36 overexpression extends cyclin D1 half-life by 2- to 3-fold, accelerating cell cycle progression. Clinically, USP36 and cyclin D1 co-expression correlates with shorter progression-free survival in cancers, supporting combined targeting strategies.

During mitosis, USP36 temporally regulates spindle assembly. Released from the nucleolus, it stabilizes checkpoint proteins (e.g., BubR1, Mad2) via deubiquitination to ensure proper chromosome segregation. Live imaging shows USP36-deficient cells have higher chromosome misalignment rates and mitotic delays, rescued by wild-type but not nucleolar-localization-defective USP36. USP36 also modulates the anaphase-promoting complex/cyclosome (APC/C) by stabilizing Emi1, ensuring orderly mitotic progression. These functions position USP36 as a key node in cell cycle quality control.

USP36 influences cellular senescence by regulating ribosome biogenesis and translation efficiency, indirectly affecting the senescence-associated secretory phenotype (SASP). In oncogene-induced senescence (OIS) models, USP36 expression declines with senescence progression, while its overexpression partially bypasses senescence. Metabolomics show USP36 maintains mitochondrial proteome renewal via ribosome function, preventing senescence-linked mitochondrial dysfunction. These findings highlight USP36 as a potential target for senescence-related diseases.

USP36's Dual Role in Tumorigenesis

In MYC-driven lymphomas, USP36 stabilizes MYC protein by deubiquitination, enhancing its transcriptional activity. Proteomics reveal USP36 binds the MYC-MAX complex, protecting MYC from FBW7-mediated degradation. In Burkitt lymphoma cells, USP36 overexpression triples MYC half-life, driving proliferation and metabolic reprogramming. Hematopoietic-specific USP36 knockout delays lymphoma onset in MYC-transgenic mice, offering new therapeutic insights for MYC-dependent cancers.

In colorectal cancer, USP36 stabilizes β-catenin to amplify Wnt signaling. Immunohistochemistry shows USP36 expression correlates with nuclear β-catenin accumulation during progression. USP36 knockout reduces β-catenin target genes and tumor stem cell markers by 70%. Notably, in APC-mutant organoids, USP36 inhibition synergizes with 5-FU, increasing tumor regression rates from 30% (monotherapy) to 80% (combination). These findings support combinatorial therapeutic approaches.

USP36 reprograms tumor metabolism by supporting nucleotide synthesis and global protein synthesis, particularly for glycolytic and glutaminolytic enzymes. Isotope tracing shows USP36 inhibition reduces glucose uptake by 50% and glutamine utilization by 60%, sensitizing tumors to energy stress. Targeting USP36 may thus offer a novel strategy to modulate the tumor metabolic microenvironment.

Therapeutic Prospects and Challenges for Targeting USP36

Structure-based USP36 inhibitor design has identified benzimidazole derivatives like BI-U36-1 (IC50 = 1.8 μM; >40-fold selectivity). Optimized candidate CC-12036 achieves 65% tumor growth inhibition in MYC-high lymphoma models without hematopoietic toxicity.

RNA interference strategies (e.g., siRNA/shRNA) show anti-proliferative effects in tumor models. Nanoparticle-encapsulated USP36 siRNA exhibits tumor-selective delivery in liver cancer xenografts, reducing USP36 mRNA by 80% for 5–7 days post-dose. CRISPR-Cas9-mediated USP36 knockout induces sustained proliferation arrest and chemosensitization in vitro. However, delivery efficiency and safety remain challenges for genetic approaches.

Key hurdles include:

·          Nucleolar delivery barriers: Most small molecules cannot access the nucleolus. Solutions include nuclear/nucleolar-targeting peptide-modified nanocarriers.

·          Biomarker development: Potential predictors include rRNA processing intermediates, ribosomal protein ubiquitination status, and MYC stability. Proteomic signatures of USP36-high tumors may guide patient stratification.

As USP36 biology is further unraveled, targeting this nucleolar regulator may yield innovative therapies for diverse malignancies.

Click on the product catalog numbers below to access detailed information on our official website.

Product Information